Optical fiber with nano-particle overclad

ABSTRACT

An optical fiber includes a core, a cladding layer, and an overclad layer that has a plurality of nano-particles the core.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/884,463, filed on Jun. 20, 2001, entitled OPTICAL FIBER WITHNANO-PARTICLE CLADDING, which is incorporated by reference herein in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to optical fibers, and more particularly, tooptical fibers with an overclad layer having a low Young's modulus andhigh water repellency characteristics.

2. Discussion of the Related Art

In the last several decades, fiber optic communication has becomeubiquitous. Fiber optic manufacturing technology has matured, andmillions of miles of fiber have been laid all over the world, connectinga variety of communication devices. An optical fiber operates bytransmitting coherent light, such as light from a modulated laser diode,down its glass core. The cladding, which is around the glass core, has alower refractive index than the core, allowing light from the laserdiode to propagate down the core. Around the cladding of a conventionalfiber is the overclad.

The cladding has to perform a number of functions. One purpose of havingthe cladding around the glass core is to protect the fiber core fromvarious environmental hazards, such as water penetration, or frommicro-cracking, which degrades the performance of the fiber, andtherefore degrades the transmission properties of the entire fiber opticsystem. The cladding protects the fiber from moisture and from potentialhumidity penetration through the cladding, since such moisturepenetration can generate defects in the fiber, particularly whenmechanical stresses are applied to the fiber. These mechanical stressescan include tensile stress, bending, and twisting of the fiber as thefiber is laid. It is therefore desirable to have a cladding with as lowa Young's modulus as possible, to enable the fiber to be as flexible aspossible. At the same time, it is desirable to have a cladding that ishydrophobic, so that moisture and water do not penetrate the claddinginto the fiber core.

The function of the overcladding is mechanical protection of the fibercore from external impacts and hazards, while the primary function ofthe cladding is optical (i.e., providing a medium of a lower index ofrefraction than the fiber core).

Silica glass is by far the best fiber optic material today. Silica glassis frequently used as both the fiber core material (often in the form ofdoped silica), as well as for cladding (as “regular” silica, or undopedsilica). Silica glass is also often used as the “overcladding” material.However, silica glass, despite its widespread use, suffers from a numberof disadvantages. One major disadvantage is insufficient fractureresistance (often referred to as “static fatigue,” or a short time untildelayed fracture appears after an appreciable thermal or mechanical loadis applied to the fiber).

Another shortcoming of the silica glass material is moisturesensitivity, since even a very minute amount of moisture on the fibersurface can result in a sudden failure, particularly in the presence ofsurface micro-cracks. Yet another major shortcoming of the silica glassmaterial is its inability to withstand appreciable deformations due totension or bending.

In addition to the use of silica glass claddings, other coatings areapplied to the fiber core and cladding. These include non-hermetic, orpolymer, coatings; and hermetic (carbon or metal) coatings. Polymercoatings have an advantage in that they have fairly low Young's moduli,and are therefore able to withstand large tensile or bendingdeformations. Polymers, however, have a major disadvantage in that theyabsorb moisture. Thus, long-term reliability of a polymer-coated fiberis believed to be suspect. Hermetic coatings, (e.g., metallic coatings)on the other hand, provide good protection from moisture, but have amajor disadvantage in being subject to corrosion, and fairly highYoung's moduli, which results in poor performance when mechanicalstress, such as bending or twisting, is applied. Phosphorous nickel, forexample, has a Young's modulus that is approximately three times higherthan the Young's modulus of silica glass. Due to its very high Young'smodulus, it is not uncommon for the coating, rather than for the silicaglass in the fiber core, to fail first, when subjected to mechanicalloading.

Accordingly, there is a need for a cladding material that overcomes thedisadvantages described above, i.e., in a material that has a lowYoung's modulus, has good mechanical stability, and at the same time ishermetic or hydrophobic, so as to prevent moisture from penetrating intothe fiber.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an optical fiber withnano-particle cladding and a method for manufacturing same thatsubstantially obviates one or more of the problems due to limitationsand disadvantages of the related art.

An object of the present invention is to provide a fiber optic structurewith a high resistance to water, high flexibility and increasedefficiency.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, in oneaspect of the present invention there is provided a optical fiberincluding a core, and a cladding layer including a plurality ofnano-particles around the core.

In another aspect of the present invention there is provided an opticalfiber bundle including a plurality of cores, and a cladding layerincluding a plurality of nano-particles, wherein the plurality of coresare embedded within the same cladding layer.

In another aspect of the present invention there is provided an opticaltransmission structure including a substrate, a waveguide formed on thesubstrate, and a cladding layer including a plurality of nano-particlesover the waveguide.

In another aspect of the present invention there is provided an opticaltransmission structure including a substrate, a plurality of waveguidesformed on the substrate and a cladding layer including a plurality ofnano-particles over the waveguides and between the waveguides.

In another aspect of the present invention there is provided a method ofmanufacturing a fiber structure including the steps of forming a fibercore, and coating a fiber core with a cladding layer that includesnano-particles.

In another aspect of the present invention there is provided a method ofmanufacturing a light transmission structure comprising the steps offorming a waveguide on a substrate, and coating the waveguide with acladding layer that includes nano-particles.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 is a schematic for purposes of quantitatively illustrating theadvantages of small-radius high-Young's modulus structures;

FIG. 2 illustrates one embodiment of the optical fiber of the presentinvention;

FIG. 3 illustrates another embodiment of the optical fiber of thepresent invention;

FIG. 4 illustrates yet another embodiment of the present invention withmultiple fiber cores embedded in the same cladding-coating; and

FIG. 5 illustrates yet another embodiment of the present invention withmultiple layers of dissimilar nano-particles in the same cladding.

FIG. 6 shows the smart material used in the overclad layer.

FIG. 7 shows the overclad layer comprising an inner hydrophilicnano-particle layer and an outer hydrophobic nano-particle layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

As an initial matter, it is desirable to have a fiber of small radiusand low Young's modulus. This may be shown as follows.

In order to quantitatively assess the gain in terms of the inducedstress that could be expected as a result of the application of alow-modulus, low CTE (coefficient of thermal expansion) and/orcoating/cladding layer, consider a coated optical glass fiber,manufactured at an elevated temperature and subsequently cooled down toa low (for example, room temperature), such as that shown incross-section in FIG. 1.

The longitudinal interfacial displacements, u₀ (z) and u₁ (z), of theglass fiber and its coating, respectively, can be, in an approximateanalysis, expressed by the equations: $\begin{matrix}{{{u_{0}(z)} = {{{- \alpha_{0}}\Delta\quad{tz}} + {\lambda_{0}{\int_{0}^{z}{{T_{0}(z)}{\mathbb{d}z}}}} - {\kappa_{0}{\tau_{0}(z)}}}}{{u_{1}(z)} = {{{- \alpha_{1}}\Delta\quad{tz}} + {\lambda_{1}{\int_{0}^{z}{{T_{0}(z)}{\mathbb{d}z}}}} - {\kappa_{1}{\tau_{0}(z)}}}}} & {{Equations}\quad(1)}\end{matrix}$

where α₀ and α₁ are the CTE of the glass and the coating/claddingmaterials, respectively, and Δt is the change in temperature,$\begin{matrix}{\lambda_{0} = {{\frac{1}{\pi\quad E_{0}r_{0}^{2}}\quad{and}\quad\lambda_{1}} = \frac{1}{\pi\quad{E_{1}\left( {r_{1}^{2} - r_{0}^{2}} \right)}}}} & {{Equation}\quad(2)}\end{matrix}$

are the axial compliances of the glass and the coating, where E₀ and E₁are Young's moduli of these materials, r₀ is the glass fiber radius; r₁is the outer radius of the coating, $\begin{matrix}{{T_{0}(z)} = {2\pi\quad r_{0}{\int_{- l}^{z}{{\tau_{0}(z)}{\mathbb{d}z}}}}} & {{Equation}\quad(3)}\end{matrix}$

is the axial thermally induced force, T₀ (z) being the interfacialthermally induced stress, and 1 is half the fiber length,$\begin{matrix}{\kappa_{0} = {{\frac{r_{0}}{E_{0}}\quad{and}\quad\kappa_{1}} = {\frac{r_{0}}{2E_{1}}\left\lbrack {{\frac{4\left( {1 + v_{1}} \right)}{1 - \gamma^{2}}\ln\sqrt{\frac{1 + \gamma^{2}}{1 - \gamma^{2}}}} - 1} \right\rbrack}}} & {{Equation}\quad(4)}\end{matrix}$

are the interfacial compliances of the glass and the coating,respectively, γ=r₀/r₁ is the radii ratio, ν₁ is Poisson's ratio of thecoating material.

The origin of the coordinate z is in the mid-cross-section of the fiberat its axis. The first terms in Equation (1) are stress-free(unrestricted) thermal contractions of the materials, the second termsare the displacements caused by the thermally induced forces T₀ (z).These are obtained on the basis of Hooke's law and reflect an assumptionthat the thermally induced displacement due to the forces T₀ (z) are thesame for all the points of the given cross-section z of the givenmaterial. The third terms in Equation (1) are, in effect, corrections tothe second terms. These corrections account for the fact that the givencross-section of the coated fiber does not remain flat, and that theinterfacial (longitudinal) displacements are somewhat larger than thelongitudinal displacements of the other points of the cross-section. Thethird term reflects an assumption that the correction in question can befound as the product of the interfacial compliance and the level of theinterfacial shearing stress in the given cross-section and are notaffected by the stresses and strains in the adjacent cross-sections.

The boundary condition u₀ (z)=u₁ (z) of the longitudinal interfacialdisplacements yields: $\begin{matrix}{{{{\kappa\tau}_{0}(z)} - {\lambda{\int_{0}^{z}{{T_{0}(z)}{\mathbb{d}z}}}}} = {{\Delta\alpha}\quad\Delta\quad t\quad z}} & {{Equation}\quad(5)}\end{matrix}$where κ=κ₀κ₁ and λ=λ₀λ₁   Equation (6)

are the total interfacial and the total axial compliances, respectivelyandΔα=α₁−α₀   Equation (7)

is the difference in the CTE of the coating and the glass.

Differentiating Equation (5) with respect to the coordinate z, we have:κτ₀ ¹(z)−λT ₀(z)=Δα Δt   Equation (8)Since T ₀(±l)=0   Equation (9)

the interfacial shearing stress function τ₀ (z) should satisfy theboundary condition $\begin{matrix}{{\tau_{0}^{/}(l)} = \frac{{\Delta\alpha}\quad\Delta\quad t}{\kappa}} & {{Equation}\quad(10)}\end{matrix}$

The next differentiation of equation (8) yields: $\begin{matrix}{{{\tau_{0}^{//}(l)} - {k^{2}{\tau_{0}(z)}}} = 0} & {{Equation}\quad(11)} \\{{{where}\quad k} = \sqrt{2\pi\quad r_{0}\frac{\lambda}{\kappa}}} & {{Equation}\quad(12)}\end{matrix}$

is the parameter of the interfacial shearing stress.

Equation (11) has the following solution:τ₀(z)=C ₀ sin h kz+C ₁ cos h kz   Equation (13)

The function of τ₀ (z) must be anti-symmetric with respect to themid-cross-section z=0 and therefore one should put C₁=0. As to theconstant C₀, it can be determined from Equation (10) as follows:$\begin{matrix}{C_{0} = {\frac{\Delta\quad\alpha\quad\Delta\quad t}{k\quad\kappa} = {\frac{k}{2\pi\quad r_{0}}\frac{\Delta\quad\alpha\quad\Delta\quad t}{\lambda}\frac{1}{\cosh\quad{kl}}}}} & {{Equation}\quad(14)} \\{{{so}\quad{that}\quad{\tau_{0}(z)}} = {\frac{k}{2\pi\quad r_{0}}\frac{\Delta\quad\alpha\quad\Delta\quad t}{\lambda}\frac{\sinh\quad{kz}}{\cosh\quad{kl}}}} & {{Equation}\quad(15)}\end{matrix}$

Introducing this solution into the formula (3) we obtain the followingexpression for the induced force T₀ (z): $\begin{matrix}{{T_{0}(z)} = {{- \frac{\Delta\quad\alpha\quad\Delta\quad t}{\lambda}}\left( {1 - \frac{\sinh\quad{kz}}{\cosh\quad{kl}}} \right)}} & {{Equation}\quad(16)}\end{matrix}$

The maximum shearing stress occurs at z=±l, and the maximum thermallyinduced forces occur in the mid-portion of the coated fiber, and, basedon Equations (15) and (16) are: $\begin{matrix}{{\tau_{\max} = {\frac{k}{2\pi\quad r_{0}}\frac{\Delta\quad\alpha\quad\Delta\quad t}{\lambda}\tanh\quad{kl}}}{T_{\max} = {{- \frac{\Delta\quad\alpha\quad\Delta\quad t}{\lambda}}\left( {1 - \frac{1}{\cosh\quad{kl}}} \right)}}} & {{Equation}\quad(17)}\end{matrix}$

For sufficiently long (large 1 values) and/or stiff (large radius)assemblies, these equations yield: $\begin{matrix}{\tau_{\max} = {{\frac{k}{2\pi\quad r_{0}}\frac{{\Delta\alpha\Delta}\quad t}{\lambda}\quad{and}\quad T_{\max}} = {- \frac{{\Delta\alpha\Delta}\quad t}{\lambda}}}} & {{Equation}\quad(18)}\end{matrix}$

Considering Equations (2), (4) and (12), these Equations can be writtenas $\begin{matrix}{\tau_{\max} = \frac{E_{1}{\Delta\alpha\Delta}\quad t}{\sqrt{\left( {{2\quad\frac{E_{1}}{E_{0}}} + {\frac{4\left( {1 + v_{1}} \right)}{1 - \gamma^{2}}\ln\sqrt{\frac{1 + \gamma^{2}}{1 - \gamma^{2}}}} - 1} \right)\left( {\frac{E_{1}}{E_{0}} - \frac{\gamma^{2}}{1 - \gamma^{2}}} \right)}}} & {{Equation}\quad(19)} \\{T_{\max} = \frac{\pi\quad E_{1}r_{0}^{2}{\Delta\alpha\Delta}\quad t}{\frac{E_{1}}{E_{0}} + \frac{\gamma^{2}}{1 - \gamma^{2}}}} & {{Equation}\quad(20)}\end{matrix}$

These equations indicate that both the shearing stress and the thermallyinduced force can be reduced substantially, if low Young's modulescoating materials are used, as well as materials with good thermal matchwith the silica glass and low manufacturing temperature. Equations (19)and (20) indicate also that the induced stress and force decrease with adecrease in the thickness of the coating/cladding, i.e. the inducedstresses and forces are smaller the closer the radii ratio are to one.

FIG. 2 illustrates one preferred embodiment of the present invention.

As may be seen from FIG. 2, a fiber core 100 is surrounded by aheterogeneous cladding layer 110, with the cladding layer 110 beingcomprised of very small nano-particles and, optionally, a liquid orsemi-liquid filler. The filler may be, for example, poly-siloxane.Alternatively, any of a number of materials whose properties are stableover time may be used, such as, e.g., synthetic oils or lubricants, forexample, chlorinated naphthalene that form a wax or oil. Such materialsare used for condenser impregnation in moisture-, flame- and/oracid-proofing of wire and cable.

The filler can be either polymerizing or non-polymerizing. For example,a low-molecular weight polyethylene with different filler may be used.Such polyethylene materials are used for packing of nuclear equipment at400 bar and 250° C. The density of the nano-particles within the filleraffects its viscosity and its Young's modulus.

Due to the presence of the nano-particles in the cladding layer 110, thecladding layer 110 is substantially hydrophobic and thixotropic.

Thixotropy is a property exhibited by certain gels of becoming fluidwhen stirred or shaken, and then returning to semi-solid state when thestirring or shaking ends. Thus, there are gels that become liquid whenshaken or stirred, but return to a semi-solid state when they areallowed to settle. These gels show thixotropy, or the property offluidity under stress. Reverse thixotropy refers to a reconstruction ofgel structure and liquid transforming to semi-solid state after the endof stress action. The semi-solid state remains for as long as the stressis less than critical level.

Due to its thixotropicity, the cladding material has a self-curingfeature (or, more precisely, nano-particles form and repair the claddinglayer due to a reverse thixotropy transformation). In other words, ifmicro-cracks or other defects develop under the surface of the cladding,then the thixotropic nature of the cladding will result in a filling ofthe cracks or defects with the cladding material. Given the size of thenano-particles (usually in the range of 5-150 nm in diameter, andtypically approximately tens of nanometers in diameter), even very smalldefects or micro-cracks in the fiber core 100 can be self-cured,therefore preventing their propagation and growth. In one preferredembodiment, the nano-particles may be hydrophilic (for example, made ofsilica glass, although other types of glasses may be used), and thefiller material is hydrophobic. The hydrophobicity is for protectionfrom water and moisture, while the hydrophilicity is for trapping andexpulsion of water and/or moisture that has already penetrated into thecladding layer 110.

Another option is the use of different types of nano-particles withinthe same filler material. One possibility of the use of hydrophobicsilica particles in combination with alumina, ceramic or metallic oxide,for example, titanium oxide (TiO₂) particles. In fact, any metallicoxide (magnesium oxide, e.g.) can also be used as hydrophilicnano-solids.

Such a combination will have different viscoelastic properties from acladding layer that only has one type of nano-particles (for example,only silica glass nano-particles). One of the advantages of thenano-particle cladding layer 110, as discussed above, lies in the factthat the cladding layer 110 of the present invention can act as both acladding and a coating, with the nano-particles together with the fillerfunctioning as both a cladding and a coating of a conventional fiberstructure. The combination of metal, metal oxides and silicanano-particles allows getting the predetermined optical characteristicsof the boundary between fiber core surface and covering. It improves anoptical efficiency of fiber. The desired properties of contact layer maybeen achieved through thermal, electromagnetic or optical treatment.

Another option is the use of two layers of dissimilar nano-particleswithin the same cladding layer 110, as illustrated in FIG. 5. The innerlayer 115, close to the fiber core 100, may be comprised of hydrophilicmetallic oxide nano-particles (or hydrophilic molybdenum disulfide),while the outer layer 116 may be comprised of hydrophobic silicanano-particles. Here, other possible materials for nano-particlesinclude molybdenum disulfide for use as the inner layer 115 with goodadhesion to the silica glass core surface and very small forces offriction. Hydrophobic particles of Teflon or other synthetics may beused as an outer layer 116. Alternatively, Teflon may be used as afiller itself. Water cannot enter to pores of hydrophobic materials ifsize of pore less 0.1 micron at pressure more 20.0 MPa. This allows usehydrophobic particles as filler while providing a very small contactsurface in contact with the fiber core while preserving optimalproperties of the cladding.

Additionally, the effective refractive index of the cladding layer 110with the nano-particles (and using air for filler) can be close to 1,due to minimal contact area between the nano-particles and the fibercore 100.

Even if a polymer is used as the filler material, the refractive indexof the polymer is less than the refractive index of the silica glass inthe fiber core 100. Thus, a major advantage of the present invention isthe ability to have a sufficient index of refraction difference betweenthe fiber core 100 and the cladding layer 110 (such as whenpolymer-based claddings are used) without sacrificing the hydrophobiccharacteristics of the silica claddings found in conventional opticalfibers. Alternatively, the filler may be air, with the nano-particlesbeing closely spaced to each other. Thus, the index of refraction ofsuch a cladding layer 110, which comprises primarily nano-particlesthemselves, is fairly close to unity, while the index of refraction ofsilica glass in the fiber core 100 is typically about 1.5-1.7.

The present approach allows a substantial simplification of themanufacturing process of making fibers. In conventional fibers, forexample, the fiber core is frequently doped in order to increase itsindex of refraction, and particularly the difference in the index ofrefraction between the fiber core and the cladding material, so as toimprove light propagation properties of the fiber. With the index ofrefraction of the cladding layer of the present invention, there issubstantially less need to dope the fiber core 100 in order to increaseits index of refraction.

Furthermore, there is no need for a separate overclad layer around thecladding, since the cladding material as described here can perform thefunctions of both the cladding and the overclad. (The overclad layer maystill be optionally added, however, as discussed below.)

As illustrated in FIG. 3, the fiber core 100 is surrounded bynano-particles in the cladding layer 110, and is in turn optionallysurrounded by a layer of an outer cladding 120 that comprises a highlyviscose polymer (for example acetate polymers based on acetic acid(alpha or octyl-cyano-acrylates, chlorinated naphthalene and othercompounds that are insoluble in water and acid solutions), or any of anumber of polymers commonly used for protective overclad layer, e.g.,BORDEN™.

As noted above, the cladding material is preferably heterogeneous, andincludes inorganic solid nano-particles with a high Young's modulus, aswell as (optionally) a liquid or a quasi-liquid inorganic filler. If thenano-particles are comprised of silica, they can be either hydrophilic(regular silica) or hydrophobic (modified silica, with methyl group CH₃on its surface). The filler material is normally hydrophobic.

In another embodiment, an optical fiber core has a hydrophilic surfacethat is covered by “dry lubrication”, for example, molybdenum disulfideand hydrophobic silica nano-particles over the molybdenum disulfidenano-particle layer. The molybdenum disulfide nano-particle layer formsa porous structure that may be protected by Teflon, polyethylene orother elastic cover. Teflon has an extremely low Young's modulus. Inthis case, Teflon creates a “sleeve” and does not fill the space betweenthe nano-particles.

In another embodiment, an optical fiber core has hydrophobic surfacethat is covered by “dry lubrication”, for example, molybdenum disulfidewith nano-powder of aluminum and hydrophobic silica nano-particles. Themolybdenum disulfide and silica nano-particles form a porous structurethat may be protected by Teflon, polyethylene or other elastic covering.The structure therefore has the silica core in the center, and an outer“coating,” with nano-particles which serve as both spacers (washers)between the fiber core 100 and the outer coating, and as ‘bearingrollers.”

In another embodiment, an optical fiber core has a hydrophilic surfacethat is covered by resin foam with “dry lubrication,” for example,molybdenum disulfide and hydrophobic silica nano-particles. Such resinfoam may be created by cyano-acrylate and/or other adhesives withorganic solvents and fillers (silica, aluminum, silver powder, etc.).This porous structure that may be protected by Teflon, polyethylene orother elastic covering.

In another embodiment, an optical fiber core has a hydrophilic surfacethat is covered by resin foam with “dry lubrication,” for example,molybdenum disulfide and hydrophobic silica nano-particles. The resinfoam may be created by evaporation of the adhesive organic solvents andmay include nano-particles of silica, aluminum nano-powder, etc. Thisstructure that may be protected by Teflon, polyethylene or other elasticcovering.

In another embodiment, an optical fiber core has a hydrophobic surfacethat is covered by resin foam with “dry lubrication”, for example,molybdenum disulfide and hydrophobic silica nano-particles. The resinfoam may be created by evaporation of adhesive organic solvents and mayinclude nano-particles of silica, aluminum powder, etc. This structurethat may be protected by Teflon, polyethylene or other elastic covering.Evaporation may be accomplished by heating using, for example,high-frequency electromagnetic field.

An optical fiber core with hydrophobic or hydrophilic surface that iscovered by resin foam with “dry lubrication”, for example, molybdenumdisulfide and hydrophobic silica nano-particles. Such foam may becreated by gas release during the chemical reaction and polymerizationof the adhesive that takes place surround of the fiber glass coresurface. This structure that may be protected by Teflon, polyethylene orother elastic covering.

Alternatively, the resin foam may result from over-saturation by aninert or neutral gas of the organic solvent or mix before or duringpolymerization of the covering (coating). This structure that may beprotected by Teflon, polyethylene or other elastic covering.

It is worth noting that hydrophobic nano-particles typically have asmall adhesion to glass fiber surface and distribute along corn due tomutual repulsion of the similarly charged particles. The chargingresults of the interaction with filling and/or outer covering fromTeflon, polyethylene or special adhesive compositions.

With the use of hydrophilic and hydrophobic silica nano-particles, incombination with a hydrophobic filler in the same cladding layer 110, a“smart” material may be formed. This is done by varying thecombinations/percentages/types of the hydrophilic and hydrophobicnano-particles, in combination with the filler material that is alwayshydrophobic. Thus, the “smart material” may be hydrophobic enough tokeep out the environmental moisture, and is at the same time hydrophilicenough to absorb and subsequently expel the already ingressed moisture.It is believed that a range of 20%-80% hydrophilic-hydrophobic to80%-20% hydrophilic-hydrophobic is useful, more preferably a range of40%-60% to 60%-40% hydrophilic-hydrophobic. As noted above, theviscoelastic properties of the heterogeneous smart material aredetermined, in part, by the density and nature of the nano-particles andthe filler.

The smart material of the present invention has a very low Young'smodulus, while at the same time having very high strain-at-failurecharacteristics, as well as a low coefficient of thermal expansion(CTE).

Additionally, the smart material of the present invention has very goodadhesion to the adjacent material, i.e., to the material of the fibercore 100, which it covers and protects. Thus, the smart material canhave a very long, practically unlimited, service life, since it is notprone to corrosion, aging, or crack and micro-crack initiation. Thesmart material of the present invention also possesses the highlydesirable property of thixotropy. This results in the ability of thesmart material to “self cure,” i.e., to restore its properties when amicro-crack attempts to initiate due to external mechanical loads beingapplied to the structure. The smart material of the present invention isalso able to “cure” the material that it coats i.e., the silica glassfiber core 100. This is due to the fact that the smart material canfill, or penetrate into, all the defects, pores, cracks, micro-cracksand voids that may exist or may be initiated in the fiber core 100.Additionally, the smart material of the present invention has an abilityto absorb mechanical energy, which may be important in applications suchas portable electronics, which are often subjected to mechanical shocksand vibrations.

A number of methods exist to manufacture the fiber core 100 with thenano-particle containing cladding layer 110 of the present invention.

One method may (1) electrically charge the surface of the glass fibercore 100 with static electricity.

The fiber core 100 may then (2) be exposed to an atmosphere saturatedwith nano-particles. Note that sedimentation of the nano-particles willoccur, with the smaller diameter particles sedimenting out first,followed by a sedimenting out of a mixture of larger-diameter particlesand small diameter particles. If different layers of different types ofnano-particles are desired, this step may be repeated.

Then, for a short period of time, (3) an electrically conductivepolymer, which contains acetic acid, is applied to the fiber.

Next, (4) a layer of smart material is applied to the surface of thepolymer.

Next, (5) the entire structure, which is wetted by an electricallyconductive polymer, is introduced into an alternating electromagneticfield (for example, RF/microwaves), in order to generate electricalcurrents inside the wetted polymer layer.

The generation of the electrical currents in the wetted polymer layerspeeds up polymerization, as well as enhances water evaporation.

The process of inducing electrical currents inside the wetted polymerlayer may be tailored so that its intensity correlates with the rate ofthe polymerization process, and the induced electrical currents drop tozero when full polymerization finally takes place.

It should be noted that when it is desirable to dispense with a fillermaterial altogether, electrostatic adhesion might be used. In this case(using the two dissimilar nano-particle layers example, where theoverall structure includes silica core/metallic oxidenano-particles/silica nano-particles, the silica core is chargednegatively, the metallic oxide nano-particles in the inner claddinglayer are charged positively, and the silica nano-particles in the outercladding layer are charged negatively. Thus, without any requirement fora filler material, the overall fiber will be highly stable, since thestatic charge is trapped in the materials.

Furthermore, the manufacturing process may take place during the drawingof the optical fibers, or it may be conducted as a separate process. Forexample, with optical fiber interconnects, the manufacturing process canbe easily modified to apply the smart material concurrently with thestripping-out of the polymer coating off the fiber (such as whensplicing and connecting two fibers together).

The nano-particles themselves may be manufactured by any number ofmethods, such as grinding, or chemical methods.

In another preferred embodiment, the following steps for themanufacturing of the smart material may be followed:

(1) A 0.5-1.0% water-glycerin solution is prepared (alternatively, analcohol or organic solvent polymer solutions or colloidal aceticacid-based raw polymer solution may be used). Glycerin, being a type ofalcohol, easily dissolves in water, while at the same time acts as abridging element to a hydrophobic surface. Also, alcohols are generallyuseful for this purpose, because they allow mixtures of materials thatotherwise have difficulty mixing, to exist at least temporarily. Thus,any number of alcohols or spirits, for example, white spirit(turpentine), mixes of alcohol with organic solvents, for example,benzyl, acetone, etc. can be used to delay the development ofhydrophobicity in the material of the cladding layer 110, until aparticular manufacturing step is completed. Such solvents or their mixreplace capillary water and may be removed from the fiber, along withresidual moisture by azeotropic evaporation by high-frequency heating,for example. It is important to note that the medium is usually notentirely dry. A small water amount in the spaces of the highly dispersehydrophobic medium with the hydrophobic nano-particles is permitted. Inthis case, water is in the form small droplets, covered on their surfaceby a thin layer of hydrophobic nano-particles, and, therefore, cannotcause any harm to the silica glass. On the other hand, these dropletsserve as elastic small balls, which provide an additional protectionfrom external environmental hazards, particularly under elevatedpressure. In this case, deformed water droplets surrounded bynano-particles block any possible paths to the glass, preventing anycontact with the aggressive external environmental hazards.

(2) The hydrophobic nano-particles are then placed into the preparedwater-glycerin solution, and are blended in a high-speed blender with ahigh vacation rate. This enables a creation of a highly heterogeneouswater-air (water-glycerin-air) medium containing a desired density ofnano-solids. A gas may also be used, instead of air.

(3) The fiber is then immersed in the water-air-glycerin solution, andremains there for about 20-40 seconds, to be exposed to the alcohol ororganic solvent polymer solutions or colloidal water-glycerin-airsolution containing the nano-particles.

(4) The wetted fiber is then exposed to microwave radiation. The waterabsorbs the microwave radiation, causing rapid azeotropic evaporation,in turn followed by an expulsion of the nano-particles out of the waterand organic solvent, which will cluster on the glass surface. Suchtreatment provides a change of the elastic and optical properties of thefoam and increases of the porosity of the foam on the surface of core.Nano-particles improve reflective properties and stability of foam,increasing its hardness. Elastic foam prevents or decreases a danger ofimpact failure. This process will create a polymer cladding on thestructure that includes the fiber core and the nano-particle “solidlayer” around it. The overall structure is therefore mechanically stabledue to the glass core-nano-particles-polymer assembly. The combinedeffect of the polymeric film and the hydrophobic nano-particles alsoresults in adequate environmental protection.

Another possible method of manufacturing includes forming a pastecontaining a polymer and the nano-particles, and drawing the fiber corethrough the paste such that the nano-particles stick to the surface ofthe fiber core.

It should be noted that liquid water generally cannot get through gapsor cracks that are smaller than 0.1 micron. Since the nano-particles, inthe preferred embodiment, are substantially smaller than 0.1 micron,typically on the order of tens of nanometers, water particles aretherefore unable to get through the gaps between the nano-particles, andthe layer of nano-particles acts as a hydrophobic layer.

An additional advantage of the present invention is the extremely smalldiameter of the overall fiber structure. In typical fibers, the fibercore 100 itself is on the order of a few microns, typically about 5-8microns in diameter, however, the cladding and the coating inconventional fibers is on the order of 125 microns in diameter. With theuse of the materials and structure of the present invention, the overalldiameter of the fiber may be substantially less than the typical 125microns, down to as low as an order of magnitude, as low as 12-15microns.

Additionally, the very small diameter of the fiber that has a smartmaterial cladding allows for a very low Young's modulus value of theoverall fiber structure. Furthermore, many of the properties of thesmart material can be very similar to glass silica, which isconventionally used in fiber optics. The smart material can be used notonly as an alternative to existing coating and cladding materials, butcan replace conventional silica cladding and “overclads” (see FIG. 6,showing the smart material used in the overclad, and FIG. 7, showing theoverclad comprising an inner hydrophilic nano-particle layer 115 and anouter hydrophobic nano-particle layer 116, with the filler, which isoptional, not shown in these figures). Due to a greater index ofrefraction difference between the fiber core 100 and the cladding layer110, the attenuation of optical signal is reduced.

It will be appreciated that the technology discussed above is applicablenot only to optical fibers, but also to other areas of photonics, forexample, rectangular and planar waveguides deposited onto substrates.

Other applications include the use of fibers outside the opticalindustry, for example, the use of small-diameter fibers to reinforcestructures, or for fiber-reinforced plastics.

Another application of the present invention relates to waveguides withhigh signal intensity and medium time resolution. For example, considera thick optical fiber with a mirror coating that has close to 100%reflection up to angles of 25-30°. The critical angle for typical silicaglass is about 41°. Thus, with the use of the smart materials of thepresent invention, transmitted signal intensity may increase by 10% to30% due to improved optical propagation properties of the fiber.

It will be appreciated that the heterogeneous cladding layer 110 in FIG.2 comprises nano-particles, as well as air or gas in the spaces betweenthe nano-particles, along with a system-stabilizing component, which canbe a polymer or a liquid (for example, poly-siloxane or silicon oils).

Due to the fact that the nano-particles and the fiber core 100 havenegligible area of contact, from an optical point of view, the fibercore 100 is, for all practical purposes, in contact with the gas or airat its surface. Thus, the index of refraction seen by light propagatingdown the fiber core 100 in the cladding layer 110 is close to 1.0, inthe case where a layer of nano-particles is used without any additionalfiller.

In another embodiment, the fiber core 100 or the silica nano-particleshas a modified surface, such that methyl CH₃ groups are substituted forOH groups. This makes the surface of the silica hydrophobic. Thismodification of the silica surface addresses the possible issue ofexisting micro-cracks, because water vapor molecules cannot attach tothe surface of the silica and therefore cannot initiate glass stresscorrosion. The presence of water repellent nano-particles on the surfaceof the fiber core 100 provides an additional environmental andmechanical protection to the fiber, while at the same time creatingappropriate conditions for propagation of light down the fiber core.

Another embodiment of the present invention is shown in FIG. 4. Asillustrated in FIG. 4, a bundle of multiple fiber cores 100 may beembedded within the same filler material (cladding layer 110). Thefiller material is as described above, and may be a hydrophobic fillerincluding nano-particles. An outer cladding layer 120 surrounds thefiller layer. Due to the fact that each individual fiber core 100 isonly a few microns in diameter, the overall structure may still have asmall diameter, however, the presence of multiple fiber cores 100 withinthe same filler allows for a dramatic increase in total throughputcapacity, without increasing the physical dimensions of the entire fiberstructure. Additionally, the cores 100 in the multiple fiber corestructure, which is illustrated in FIG. 4, need not be circular. Forexample, planar or rectangular cores (waveguides) may be deposited ontosubstrates, with nano-particle layers between them. Such waveguides canbe rectangular or square, rather than circular.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to those skilled inthe art that various changes and modifications can be made thereinwithout departing from the spirit and scope thereof. Thus, it isintended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. An optical fiber comprising: a core; a cladding layer surrounding thecore; and a thixotropic overclad layer surrounding the cladding layer.2. The optical fiber of claim 1, wherein the overclad layer comprises aplurality of nano-particles embedded in a filler
 3. The optical fiber ofclaim 2, wherein the filler includes at least one of a polymer,synthetic oil, poly-siloxane and Teflon.
 4. The optical fiber of claim2, wherein nano-particles include at least one of a ceramic, silica,molybdenum disulfide, Teflon and a metallic oxide.
 5. The optical fiberof claim 4, wherein the metallic oxide is one of titanium oxide,aluminum oxide and magnesium oxide.
 6. The optical fiber of claim 2,wherein the nano-particles are hydrophilic.
 7. The optical fiber ofclaim 2, wherein the overclad layer includes an inner layer of metallicoxide nano-particles and outer layer of silica nano-particles.
 8. Theoptical fiber of claim 2, wherein the overclad layer includes an innerlayer of molybdenum disulfide nano-particles and outer layer of Teflon.9. An optical fiber bundle comprising: a plurality of cores; a claddingin which the cores are embedded; and a thixotropic overclad layer. 10.The optical fiber bundle of claim 9, wherein the thixotropic overcladlayer comprises a plurality of nano-particles embedded in a filler 11.The optical fiber bundle of claim 10, wherein the filler includes atleast one of a polymer, synthetic oil, poly-siloxane and Teflon.
 12. Theoptical fiber bundle of claim 10, wherein the nano-particles include atleast one of a ceramic, silica, Teflon, molybdenum disulfide and ametallic oxide.
 13. An optical transmission structure comprising: asubstrate; a waveguide on the substrate; a cladding layer on thewaveguide; and a thixotropic overclad layer.
 14. The opticaltransmission structure of claim 13, wherein the thixotropic overcladlayer comprises a plurality of nano-particles embedded in a filler 15.The optical fiber of claim 14, wherein the nano-particles are formed ofat least one of a ceramic, silica, Teflon, molybdenum disulfide and ametallic oxide.
 16. The optical transmission structure of claim 14,wherein the overclad layer includes a layer of hydrophilicnano-particles on the cladding layer, and a layer of hydrophobicparticles on the layer of hydrophilic nano-particles.
 17. A method ofmanufacturing a fiber structure comprising the steps of: forming a fibercore; forming a cladding layer around the fiber core; and forming athixotropic overclad layer around the cladding layer.
 18. The method ofclaim 17, wherein the coating step forms the overclad layer comprising aplurality of nano-particles embedded in a filler.
 19. The method ofclaim 17, wherein the coating step forms an inner layer ofnano-particles, and an outer layer of nano-particles, the inner andouter layers having dissimilar hydrophobicity.
 20. The method of claim19, wherein the coating step includes the step of negatively chargingthe fiber core, positively charging the inner layer, and negativelycharging the outer layer.
 21. The method of claim 19, wherein the innerlayer includes metallic oxide nano-particles, and the outer layerincludes silica nano-particles.
 22. The method of claim 18, wherein thecoating step includes the step of immersing the fiber structure in awater-alcohol medium that includes the nano-particles.
 23. The method ofclaim 18, wherein the coating step includes the steps of: applying apolymer with the nano-particles to the cladding layer; and drying thepolymer.
 24. An optical fiber comprising: a silica core; a claddingsurrounding the core; and a thixotropic coating surrounding thecladding.
 25. The optical fiber of claim 24, wherein the thixotropiccoating comprises a plurality of nano-particles.
 26. The optical fiberof claim 25, wherein the nano-particles are embedded in a filler.
 27. Anoptical fiber bundle comprising: a plurality of silica cores embedded inc cladding; and a thixotropic coating surrounding the cladding.